PSI - Issue 68

Evgeniia Georgievskaia et al. / Procedia Structural Integrity 68 (2025) 559–565 Evgeniia Georgievskaia / Structural Integrity Procedia 00 (2025) 000–000

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• HPPs provide more than 50% of the global renewable electricity generation; in some countries, the share of hydropower can exceed 90%; • HPPs provide stability of electrical networks and power systems in general by covering daily, seasonal and other peaks of energy consumption as well as fluctuations of energy output with unstable renewable generation (wind, solar etc.). To achieve this, hydraulic units (HUs) must operate in a wide range of capacities, as well as allow for long operation time in part load or overload mode, multiple and fast increases and decreases in load, frequent startups and shutdowns. As a result, additional off-design loads affect HU’s lifetime-determining components during steady state and transient conditions, see, for example Goyal et al. (2018), Mohanta et al. (2017), Liu et al. (2023), Egusquiza et al. (2016). The design stage usually fails to fully consider the impact of such loads on HU's strength, reliability, and lifetime because the amplitude and frequency spectrum of these loads depend greatly on the manufacturing, installation, and operating modes. High-frequency (HF) hydraulic loads accompany the hydraulic unit's operation throughout the operating range and cause stress pulsations in its components, as noted many researchers, for example Ru et al. (2023). Amplitudes of these pulsations are significantly lower than the amplitudes of the pulsations caused by low-frequency (LF) loads, typical for HU’s transient operating modes. Furthermore, stress intensity factor (SIF) determined in accordance with the BS 7910 (2019) recommendations and related to HF-loads is below the crack-extension-inducing threshold value. This often results in overlooking HF loads when estimating the HU’s lifetime. However, ignoring the impact of high-frequency loads on reliability and safety of HU’s operation leads to overestimated lifetime forecasts, unplanned shutdowns, premature failures of lifetime-determining components (see, for example Makhutov et al. (2020), Liu et al. (2016)), which can cause severe breakdowns of the entire unit. The hydropower industry currently lacks a universal approach which would allow predicting the destruction moment of the responsible HU’s components in actual operation at HPPs taking into account all individual characteristics, including the impact of high-frequency effects. The article presents an approach to hydraulic unit’s lifetime estimation relying on fracture mechanics methods to unit’s main components. The approach reflects the combined impact of LF and HF loads. LF loads have high amplitudes and act as a trigger for crack formation in the initial period of operation, when the HF-loads impact is negligible due to their small amplitudes. At this stage, crack growth is slow and stable, which allows controlling the crack extension by standard non-destructive metal testing methods and take prompt corrective action. During the later stages of operation, when material damage reaches a certain critical level and the SIF threshold decreases, HF loads' influence becomes apparent. The result of this is a significant increase in growth rate of hazardous fatigue cracks, which dramatically shortens maintenance intervals as well as the HU’s lifetime overall.

Nomenclature HF

high-frequency (loads, stresses)

HPP HU LF SIF RSI Т res

hydraulic power plant

hydraulic unit

low-frequency (loads, stresses) stress intensity factor rotor-stator interaction residual lifetime rotation frequency RSI frequency blade frequency Karman vortices frequency

f r

f RSI

f b f K

ℓ

crack length

K с

fracture toughness